Charged particle beams generated via interactions or decays are created with very large emittances. This makes them very difficult to manipulate, so devices and techniques to reduce their emittance are highly desirable, especially to generate beams with high intensity and high brightness. The emittance of a beam is the volume of six-dimensional phase space that its ensemble of particles occupies, and Liouville's theorem limits the ability to change it. The phase density of the particle beam is preserved along the direction of motion of the particles within the beam. All available methods for emittance reduction are similar to thermodynamic cooling, in that the emittance (temperature) of the desired object is reduced while increasing the emittance (temperature) of some other object(s).
There are several general techniques for reducing the emittance of a charged particle beam.
“Stochastic cooling” uses variable electromagnetic fields and high-speed feedback to reduce the emittance of a beam. Unfortunately it requires many seconds to achieve a significant reduction, and is thus unsuitable for beams of unstable particles (such as pions or muons, which may have an at-rest lifetime of about 26 ns or 2.2 μs respectively). It also requires large and expensive apparatus.
“Electron cooling” uses an overlapping parallel beam of electrons to transfer the emittance of the desired beam to the electron beam. This, too, requires seconds to achieve a significant reduction, and is thus unsuitable for beams of unstable particles (such as pions or muons). It also requires large and expensive apparatus.
“Ionization cooling” involves directing a beam through a material that absorbs the energy of the incident particles (i.e., an absorber), thus reducing the momentum of each particle along its momentum vector, and then re-accelerating the beam longitudinally. This has the effect of reducing the transverse momentum of each particle in the beam (transferring it to the absorber), thus reducing the transverse emittance of the ensemble of all beam particles. This can happen quickly enough for beams of unstable particles, but conventional approaches in ionization cooling require large and expensive apparatus.
“Frictional cooling” is a specific type of “ionization cooling” that operates at low velocity, below about 0.02 times the speed of light (for particle beams this is quite slow). In this regime a faster particle loses more energy in an absorber, and a slower particle loses less energy. As the particle's energy is related to its speed, by alternating multiple absorbers and regions of re-acceleration, it is possible to create a channel with an equilibrium speed: each re-acceleration is matched to the energy lost in each absorber by a particle at the desired equilibrium speed. Slower particles lose less energy in the absorbers and are accelerated up to the equilibrium speed, while faster particles lose more energy in the absorbers and are slowed down to the equilibrium speed. At the same time, ionization cooling operates to reduce the transverse emittance of the beam. This can happen quickly enough for beams of unstable particles, and with small and relatively inexpensive apparatus. Unfortunately, the acceptance (i.e., the maximum value of beam emittance able to enter) of such a channel is quite small, as the panicles must have very low speed to enter it, so to date nobody has been able to use this for beams with high intensity or high brightness.
It would be advantageous to provide a relatively compact and inexpensive device to reduce the emittance of charged particle beams.
It would also be advantageous to provide a device well-suited for beams of particles produced by the interactions or decays of other particles, such as anti-protons, pions, ions, and muons.
It would further be advantageous to provide a device which operates very quickly, permitting use for unstable particles such as muons (lifetime of about 2.2 μs).
It would further be advantageous to provide a device with a large emittance reduction factor and with relatively high transmission.
It would further be advantageous to provide a device with large acceptance and the ability to produce beams of high intensity and brightness.